(per RFC2660)
Media Type name: message
Media subtype name: s-http
Required parameters: none
Optional parameters: version, msgtype
version: The S-HTTP version number of the enclosed message
(e.g. "1.4"). If not present, the version can be
determined from the first line of the body.
msgtype: The message type -- "request" or "response".
If not present, the type can be determined from the
first line of the body.
Encoding considerations: only "7bit", "8bit", or "binary"
are permitted.
Security considerations: this is a security protocol.